Ignitron control circuits



Oct. 4, 1960 .1. J. PALEY ErAL IGNITRON CONTROL CIRCUITS 2 Sheets-Sheet 1 Filed Nov. 19, 1959 lst/22a.

TIME

Oct. 4, 1960 J. J. PALEY ETAL 2,955,233

IGNITRON CONTROL CIRCUITS Filed Nov. 19, 1959 2 Sheets-Sheet 2 CONTROL 43 Inventors:

Jam e s J. Pa I e L.; Char-Jes E Pettg,

Th e1^ Attovneg.

United States Patent() IGNITRON CONTROL CIRCUITS James J. Paley, Albany, N.Y., and 'Charles E. Rettig,

Havertown, Pa., assignors to General Electric Company, a corporation of New York Filed Nov. 19, 1959, Ser. No. 854,161

9 Claims. (Cl. 3155-234) a rod of silicon carbide or boron carbide. To tire an ignitron current is transmitted through the ignitor electrode and the cathode to produce ionization in the region -of the ignitor electrode. By reason of the ionization an arc is ignited between the anode and the cathode, if the anode potential is of the proper magnitude at the instant of ionization.

In the operation of ignitrons it has been customary to charge a capacitor from a source of direct current and to discharge the capacitor through the ignitor electrode to provide an ignitron pulse starting the conduction. Now the resistance characteristics of an ignitor electrode is subject to change due to many factors such as temperature, agitation of the mercury pool and the depth of immersion of the ignitor electrode in the pool. For reliability of ignition it is therefore necessary to provide an excess of energy storage on the ignition capacitor. In most conventional circuits the surplus energy, that remaining after ring the ignitron, appears as a potential on the ignition capacitor of opposite polarity to the initial potential. This remaining energy causes certain random variations in the next ignition pulse, both in the amplitude of the pulse and the time during the cycle when the pulse isl delivered. Y

It is an object of the present invention to provide improved -ignitron tiring circuits in which practically all of the energy stored on the ignition capacitor is delivered t the ignitor electrode during each ignition pulse so that all pulses are of more unifonn amplitude and the pulse timing is more accurate.

It is another object of the present invention to provide improved ignitron firing circuits in which the power supply is required to deliver capacitor charging currents at relatively low rates and in which, therefore, the s ize and cost of the total control circuit may be considerably reduced.

Since this invention d'oes not contemplate a sacrifice in the performance of the control circuit in its function of governing the conduction periods of an ignitron, it is a further object of the invention to provide an ignitron control circuit of comparatively small size which is nevertheless capable of delivering very high peak currents to the ignitor electrode.

It has often been Iassumed that the ignition pulse to the ignitor electrode should be maintained Ias long as possible to preserve the ignitor electrode. We have found this assumption to be only partially true, since our observations have tended to confirm that itis the Wave shape of rnost short ignition pulses which tends to destroy the ignition electrode and not the duration of the pulse.

It is therefore still another object of the present inven- 2 tion to provide an ignitron control circuit Whose ignition pulses to the ignitor electrode possess 'a novel Wave shape conducive to prolonged ignitor electrode life.

By way of a brief summary of but one embodiment of the present invention, an ignitron control circuit is provided in which an ignition capacitor receives its charge from `another capacitor connected to be charged at a comparatively leisurely rate. Periodically a conducting path is established through Ia low-rated thyratron tube connected in a common circuit loop with the two capacitors to permit the energy on the charged capacitor to transfer onto the ignition capacitor. Shortly after receiving its full charge the potential on the ignition capacitor causes a saturable reactance connected in closed series circuit with it `and the ignitor electrode of an ignitron tube to saturate, thereby providing a 10W reactance discharge path through the ignitor electrode which causes the formation of `a cathode spot and initiates the conduction period of the ignitron. The firing of the thyratron is done at intervals which are phase-related to the alternating potentials impressed across the ignitron to control the average current therethrough.- Across the ignition capacitor is connected a free-Wheeling rectifier which permits the accumulation on the ignition capacitor of a potential of the proper polarity to form a cathode spot, but which prevents the acquisition of any substantial reverse polarity by the ignition capacitor. The effect of this rectifier in combination ywith the residual inductance in the ysaturated reactor is to continue to pass currents through the ignitor electrode after the ignition capacitor has been substantially discharged. With such a circuit, the ignition pulse through the ignitor electrode is found to have a very sharp wave front, which contributes to the precision of control exercised over the tiring of the ignitron, and a gradual decay of current following the ignition, which we have found greatlyV increases the life of the ignitor electrode and improves the precision of con-V trol which may be exercised over the ignitron.

Further details of the invention 'as Well as additional objects and advantages will be more readily :apparent with reference to the following more complete description taken in connection with the accompanying 'drawings wherein:

Fig. 1 is a schematic circuit diagram of a simplitied version of an ignitron control circuit constructed in accordance with this invention; Y

Fig. 2 is a graphical representation of pearing across certain circuit elements of Fig. l;

Fig. 2r: is a graphical representation of the ignition current pulse through the ignitor electrode; and

Figs. 3 to 5 are schematic circuit diagrams of alternate embodiments of this invention incorporating certain variations and refinements in accordance with these teachings.

With reference to Fig. l there may be seen an ignitron 1 having an anode 2, a cathode 3 in the form of a pool of mercury, and an ignitor electrode 4 in contact with the cathode. While but one ignitron is shown it should be realized that the more usual case will include multiple ignitrons connected in polyphase circuit relationship, either full or half wave. Since polyphase circuit connections themselves are not part of the present invention and because many forms of them are Well known in the art, the showing of other ignitrons and their interconnecpotentials aptions have lbeen omitted to avoid unnecessary duplication which functions as an ignition capacitor. During the 'A Patented oet. 4, raso 3 half cycle of the applied alternating current potentials when the conductor 6 is .positive and the ignitron 1 is therefore non-conductive the tirst capacitor 7 is free to assume a charge. The current for'this purpose ows through a charging circuit including rectifier 10 and current limiting resistor 11. The charging of capacitor 7 takes place during an entire quarter cycle ofthe applied alternating currentpotentials, 'and the capacitor maintains its charge after reaching Vits peak potential. The rectifier 10 prevents the charge on the capacitor from following line potential after the peak potential is reached, and the switching means represented by normally non-conducting thyratron 12 prevents the capacitor 7 from draining through the discharge circuit until the desired phase angle occurs.

The discharge circuit for capacitor 7 in Fig. 1 includes on the one hand a saturable reactor 13 and, connected across it, an ignition capacitor 8 in series circuit with the ignitor electrode 4 and the cathode 3 of the ignitron. The thyratron 12 and linear reactor 14 govern the man ner in which the capacitor 8 is permitted to discharge through either of these circuit paths. When the thyratron 12 is triggered into conduction by a pulse signal applied to its control grid through the medium of a phase controllable circuit 1S, a current begins to flow in the discharge circuit, resulting in transferring most of the energy on capacitor 7 to the ignition capacitor 8. Most of the discharge current for capacitor 7 will flow through the ignitor electrode 4 in the opposite direction from that required to fire the ignitron and will develop a potential on the ignition capacitor 8. Because the saturable reactor 13 is, as'yet, unsaturated it presents a high impedance to the discharge currents and only a small, initial magnetizing current will exist in it. Since the tendency of the reactor 14 is to maintain current owing through the thyratron 12, more charge is transferred to ignition capacitor 8 than was actually on the capacitor 7. Consequently, before the thyr-atron 12 ceases to conduct because of the decline and reversal of the potential across it, the capacitor 7 will actually acquire a reverse charge.

When the potential on the ignition capacitor 8 reaches its peak the only discharge path open to it is back through the ignitor electrode 4 and the saturable reactor 13. VThese elements together consti-tute the ignition circuit. It should be noted that this direction of current iiow through the satu'rable reactor 13 is the same as that occasioned `during'the discharge of capacitor 7. After a brief time interval the magnetizing current in the saturable reactor y13 succeeds in saturating its rna-gnetic core. Consequently the reactance of this element drops to a very low value and the ignition capacitor is effectively short circuited through the ignitor electrode. The current which then flows through the ignitor electrode is of sufricient magnitude and in the proper direction to cause the formation of a cathode spot, rendering the ignitron conducting to current owing from conductorV 5 to conductor 6.

In Fig. l the free-wheeling or bypass rectifier 16 connected across the ignition capacitor 8 has two functions. One of its purposes is to permit the flow of follow-up currents in the same direction after the ignition capacitor has discharged. It should Vbe realized that, although the saturable reactor 13 presents a comparatively low irnpedance after saturation, its impedance is nevertheless appreciable and, in this case, significant. After saturation the reactor 13 possesses acertain amount of inductively stored energy and hassub'stantially the characteristics of an air core inductor which tends to keep current owing in the same direction therethrough. Without the rectier 16 Vconnected in circuit this tendency on the part of the reactor 13 would bring about apeaking of the 'reverse potential on capacitor 8, an effect which can have destructive consequences on the ignitor electrode. Rectiiier v16 preventsY such an ,e'tect by short-circuiting the current about capacitor 8, and in doing so causes a comparatively gradual decline in the ignition pulse currents instead of an abrupt termination or sudden reversal of the ignition pulse currents. As a result, substantially al1 the energy stored in the ignition circuit is dissipated after each ignition pulse.

Another function of the rectifier 16 in this particular example is to reset the flux in the magnetic core of the saturable reactor 13 and for this purpose it is selected toV have a comparatively high threshold voltage. By this is meant that it will stand off a few volts in a forward direction before beginning to conduct. Such a rectifier may conveniently be provided by connecting -a number of individual selenium cells in series until a composite rectifier possessing the desired threshold voltage is achieved. Because of its threshold voltage the rectifier 16 actually permits the accumulation of some reverse charge on the ignition capacitor 8 before it begins to conduct. But this is small compared to its full charge. The effect of the parallel combination of the rectifier 16 and capacitor 8 is therefore similar to a battery in supplying the reset voltage to the reactor 13 to drive the flux in the magnetic core in the opposite direction on the hysteresis loop, thereby to prepare the reactor for the next cycle of operation.

Fig. 2 is a graphic illustration of the potentials V7 and V8 appearing across capacitors 7 and 8 respectively. Both curves are drawn on the same time scale with the dotted line representing, for reference purposes, the sinusoidal potential appearing across conductors 5 and 6. It can be seen that during the initial degrees of the applied alternating current potential the potential on capacitor 7 follows the sinusoidal potential to a maximum or peak potential at point A. When the alternating potential declines, however, the charge on capacitor 7 remains constant until at a pre-determined point B the thyratron 12 is triggered into conduction. The potential on capacitor 7 does not drop off immediately because of the effect of the inductor 14, but rolls off gradually, its rate of discharge increasing more and more rapidly.

As the capacitor 7 discharges from point B to point C, the potential on the ignition capacitor 8 rises correspondingly from point D to a maximum at point E. Thereafter the capacitor 8 does not maintain a constant charge, since there is some drain on it to provide the magnetizing current for the saturable reactor 13. This drain results in the slope of the ignition capacitor potential from point E to point F, which represents the instant at which the saturable reactor 13 saturates.

' Immediately on the occurrence of this event the potential on the ignition capacitor drops very sharply as the capacitor supports 'an ignition pulse through the ignitor electrode. The potential on capacitor 8 swings negative by a small amount until at point G the threshold potential of rectifier 16 is exceeded. After this time the reverse potential on the ignition capacitor 8 leaks off gradually resetting the core of saturable reactor 13. When the reactor 13 saturates the drop in potential across it permits the inductor 14 to pass some additional cur- -rent through itself before the thyratron ceases to be conducting. For this reason the potential on capacitor 7 goes even 'more negative Yas evidenced by the jog in its potential curve between points C and H. This negative potential is maintained by capacitor 7 until point I, at which time'the applied sinusoidal potential catches up with it, as it were, and begins to reverse the charge again.

Fig. 2a shows on a different time scale the wave shape of the ignitor current, i.e. it shows the ignition pulse. This pulse is characterized by a very sharp rise in current represented by the almost vertical leading edge of the curve. It can be shown that from the initiation of this pulse `to just past its peak the curve is substantially sinusoidal.Y However, `just past the peak the nature of the `pulse-clrranges abruptly and insteajdof falling otf as sharply as it had risen, a follow-up current ows causing the current pulse to decline gradually to zero. As a consequence, almost all of the energy stored in the ignition circuit is delivered to the ignitor electrode, leaving the ignition capacitor in substantially identical conditions at the beginning of each cycle. This results in more uniform ignition pulses precisely timed to give accurate and reliable control over the operation of the ignitron. Furthermore, we have found that the gradual decline of the ignition pulse to zero is more conductive to prolonged ignitor electrode life than are current pulses which decline abruptly or reverse their direction suddenly in an oscillatory manner.

In the embodiment of this invention illustrated in Fig. 3 there may be seen certain variations which, among other things, permit the insulation of the charging capacitor circuit from the ignition capacitor circuit. As in the previous example the ignitron 31 is possessed of an anode 32, a cathodic vpool of mercury 33, and an ignitor electrode 34, the ignitron being connected across conductors 35 and 36 having an alternating current potential impressed upon them. The charging capacitor 37 in this example is charged from a separate source 38 of alternating current power related in phase to the alternating current potential on conductors 35 and 36. The coupling between the source 38 and charging capacitor 37 is effected through transformer 39, current limiting resistor 40 and the rectifier 41, the latter element permitting the charging capacitor 37 to hold the peak potential applied to it by the alternating current source.

As in the Fig. 1 embodiment the charging capacitor is discharged through a thyratron 42 triggered into conduction by a pulse signal applied to its control grid through the medium of a phase controllable circuit 43. When the thyratron is thus rendered conductive the charge on capacitor 37 drains off through the primary of saturable transformer 45 and through the linear reactor 46, which provides a transfer inductance. This action produces a current flow in the secondary circuit of the unsaturated transformer 45 to charge the ignition capacitor 47. Most of the current required to charge the ignition capacitor 47, instead of passing in a reverse direction through the cathode 33 and the ignitor electrode 34, is shunted through another circuit branch including a bypass rectifier 48, which is poled to permit flow of the charging current but to block the discharge current. The addition of rectifier 48 reduces the total current required to be passed through the ignitor electrode and tends to prolong the life of the ignitron still further.

As the charge on the ignition capacitor 47 builds up and even after it has reached its highest point, a magnetizing or exciting current flows in the transformer 45 until after a certain interval determined by the constants of the system the core of the transformer saturates. Thereupon the secondary of the transformer 45 becomes a low impedance path for the discharge of the ignition capacitor. A cathode spot-forming ignition pulse of the general wave shape shown in Fig. 2a is therefore passed through the ignitor electrode and the cathode, the freefunctioning, as before, to permit' the flow of a follow-up current causing a gradual decaywheeling rectifier 49 of the ignition current pulse following its initial sharp rise. Rectifier 49 is preferably constructed as was rectifier 16 in the Fig. l exampleto possess a relatively high threshold potential which, in combination with capacitor 47, connected across it, aids in resetting the flux in the core of transformer 45 to prepare it for the next cycle of operation.

For ease of identification of the respective components many of those elements of Figs. 4 and 5 corresponding to their functional counterparts in Fig. 3 have been assigned the same reference number. In the Fig. 4 modification one of the principal dierences resides in the use of a pulse-shaping circuit combination connected across ignition capacitor 50. This pulse-shaping circuit includes bypass capacitor 51, bypass rectifier 52, and an adjustable resistor 53. This circuit combination permits a measure of Vcontrol to be exercised over the width of the ignition pulse.l We have found that with values of three microfarads for capacitor 50 and 200 microfarads for capacitor 51 and a residual inductance of about 300 microhenries in the secondary winding of saturable transformer 45, an ignition pulse which would have lasted two electrical degrees at sixty cycles is stretched to approximately 5 electrical degrees. Thus the final rate of change of current in the ignition pulse is greatly decreased with a consequent decrease in the duty on the ignitor electrode.

` After the ignition pulse is completed it is necessary to return the iiux in the core of transformer 45 to its original unsaturated condition. To do this a voltage must be applied across the secondary of the transformer with a polarity opposite to that which appeared during saturation. Such a voltage is obtained from the reset circuit elements including inductor 54 and resistor 55. As a result of the fact that a portion of the ignition pulse current flows through the reset reactor 54, following the termination of the ignition pulse this reactor tends to keep a current owing through it in the same direction, and this results in a reset current owing through resistor 53 and through the secondary of transformer 45 in a reverse direction, resetting it. With a separate reset circuit of this nature rectifier 52 does not have to be of a type which possesses a high threshold potential, `since the potential across the bypass rectifier is not employed to reset the saturable reactor.

' In Fig. 5 is shown an alternative ignitron control circuit in which the ignitron 60 is supplied with a holding anode 61. The holding anode is supplied with a holding potential in a known way through a rectified D.C. supply 62 which is represented in the drawing by a lament transformer 63 coupled to a source 64 and providing full-wave rectified potentials to the holding anode through rectiers 65 and resistor 66. In this example the potential to reset the core of saturable transformer 45 is obtained from the holding anode circuit through an inductor 67 and a resistor 68. The inductor 67 serves to isolate the ignition capacitor 70 from the holding anode circuit during the time the ignition capacitor is being charged. As might be expected, the reactor 67 does draw some current from the ignition capcitor and decreases the charge upon it. For this reason reactor A67 should be as large as practical and the saturation of transformer 45 should occur as soon as possible after the voltage across the ignition capacitor reaches its peak. As in the previous example, a pulseshaping circuit comprising bypass capacitor 71, bypass rectifier 72 and resistor 73 is connected across the ignition capacitor to shape and stretch the ignition pulse applied to the ignitron through the ignitor electrode 34.

As can be seen each of the embodiments of this invention described above may employ a comparatively inexpensive thyratron vto initiate the firing pulse. 'Ihe saturable reactor permits the rise time of the ignition pulse to be Very brief, which results in precision of control and also avoids the waste of the ignition power on initial ignition currents which would be insu'icient in magnitude to form a cathode spot. Coupled with the sharp rise of current in the ignition pulse is a comparatively gradual decay of the ignition current which prevents oscillation of the current through the ignition electrode and inhibits large reverse currents which might otherwise flow through the ignition electrode. This greatly prolongs the useful life of the ignition electrode and enhances the precision of control by returning the ignition capacitor to the same condition after each ignition pulse. In addition, since the peak currents and inverse potential in the charging circuit are so low, the energy levels involved in the accumulation of charge from the source circuit are conducive to a long useful life of the thyratron.

It should be appreciated that the potential and current curves described herein are not identical for each of the circuits depicted, but are primarily representative. They are .intended to illustrate the general principles characterizing the operation of this invention rather than to show precise wave forms. It is therefore to be expected that certain 'variations will be noted in particular circuits.

At this point it will have become apparent that the various speciiic embodiments described herein are offered as purely illustrative in nature and that they should not necessarily be taken as limiting on therinveution disclosed herein. Many other modifications within the true spirit and scope of these teachings inaddition to those already depicted will doubtless occur to those'skilled in the art to which this disclosure pertains.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

l. In combination: an i'gnitron having an ignitor electrode in contact with a cathodic pool of mercury; means for impressing a source of alternating current potential across said ignitron; an ignition capacitor; a saturable reactor; a closed series circuit `connecting said capacitor and said saturable reactor in mutual series relationship with said ignitor electrode through said cathodic pool of mercury; V'means for periodically storing a charge on said capacitor and for periodically discharging said capacitor through said series circuit to cause the dow of an arciigriiting current pulse through said ignitor electrode lin response to saturation of said reactor; and a current conducting path connected across said ignition capacitor and containing a rectifier poled to effect resetting of the iiux of said saturable reactor following discharge of said capacitor and permit the lflo-w of inductively caused followup currents through said ignitor electrode after the discharge of said ignition capacitor.

2. In combination: an ignitron having an ignitor electrode :in contact with a cathodic pool of mercury; means for impressing a source of alternating current potential across said ignit-ron; an ignition capacitor; a saturable reactor; a closed Vseries circuit connecting said reactor and capacitor in mutual series relationship with said ignitor electrode through said cathodic pool of mercury; means for storing a charge on said capacitor of a polarity to cause the flow of an `arc-igniting current pulse through said ignitor electrode-in response to saturation of said reactor; a current conducting path Vconnected across -said ignition capacitor containing a rectiher poled to effect resetting of the iluX of said saturable reactor following discharge of said capacitor and permit the iiow of followup currents through said ignitor electrode after the discharge of said ignitioncapacitor.

3. In combination: an ignitron having an ignitor electrode in contact with a'cathodic pool Vof mercury; means for impressing a source of alternating current potential across said ignitron; -aiirst capacitor; means for vstoring energy in `the form of a charge on said lirst capacitor during the half Ycycle of saidyalternating current when said ignitron is non-conductive; a second capacitor; means for transferring the energy represented by the charge on said iirst capacitor onto said second capacitor at preselected intervalsrelatedfto the phasing of said alternating current source; inductance means having saturable magnetic material associated therewith to cause said inductance means to operate at diierent levels of reactance', said inducta-nce means being connected in closed series circuit with said second-capacitor and said ignitor electrode to provide -a discharge path for the charge on said second capacitor, said magnetic material saturating a 'predetermined time interval following the charging of said second capacitor to cause the impedance of said discharge path to decrease thereby to form an arc-igniting cathode spot in said ignitrori by the sudden discharge of said second capacitor; and a circuit Ybranch including a rectiiier connected across'said second capacitor and poled to permit the saturated inductance means to continue to force currents therethrough after vthe discharge of said second capacitor;

4. In combination: an ignitron having an ignitor electrode in contact with a cathodicpool of mercury; means for impressing a source of alternating current potential across said ignitron; a charging circuit including a lirst capacitor connected to receive and store anrelectrical charge; an ignition circuit including a secondrcapacitor connected in series with said ignitor electrode, a discharge circuit including switching means periodically coupling said charging circuit wit-h said ignition circuit to periodically transfer energy represented by the charge on said first capacitor onto said second capacitor; and saturable reactarice means in said ignition circuit adapted to saturate a predetermined time interval after said second capacitor assumes its charge, thereby converting said ignition circuit into a low impedance discharge path permitting flow of an ignition current pulse to said ignitor electrode. Y

5. In combination: `an ignitron having an ignitor electrode in contact with a cathodic pool of mercury; means for impressing a source of alternating current potential across said ignitron; and meansV for periodically applying through said ignitor electrode current pulses to form cathode spots in predetermined phase relationship with said alternating current potentials comprising: a saturable ieactance device having high and low impedance states and an ignition capacitor connected to formV with said ignitor electrode and said cathodic pool of mercury a closed series-connected ignition circuit, means for periodically placing a charge on said ignition capacitor thereby permitting said charge to supporta magnetizing current through said saturable reactance device causing, after a predetermined ytime interval, saturation of said device and consequent conversion of said device to its low impedance state, whereby periodic current pulses flow in said ignition circuit, and a bypass circuit branch connected in parallel across said ignition capacitor including a bypass rectifier poled to permit tthe flow of follow-up currents in the same direction as Ysaid current pulse, whereby said current pulses are characterized by a sharp rise of current followed by a relatively gradual decay of current.

6. In combination: an ignitron having an ignitor elec trode in contact with a cathodic pool of mercury; means for impressing a source of altern-ating current potential across said ignitron; a charging circuit including a first capacitor connected to receive and store an electrical charge; an ignition circuit including a second capacitor connected .in series with said ignitor electrode; a discharge circuit including switching means periodically coupling said charging circuit with said ignition circuit to periodically transfer energy represented by the charge on said first capacitor onto sai-d second capacitor; saturable react-ance means in said ignition circuit adapted to saturate Ia predetermined time interval after said second capacitor assumes its charge thereby converting said ignition circuit into a low impedance discharge path permitting ow of an ignition current pulse through said ignitor electrode, and a bypass circuit branch connected across said second capacitor including a bypass rectifier poled to permit the flow of followup currents in the same direction Vas said ignition current pulse, whereby said pulse is characterized by a sharp rise of current folloivedibyia relatively .gradual `decline of current which dissipates substantially all the energy stored in said ignitibncii'cuit.r`

' 7. In combination: an ignitron having an ignitor electrode in contact with a cathodic pool of mercury; means for impressing a source of alternating current potential across said ignitron; a charging circuit including ya iirst capacitor connected to receive and store an electrical charge; an ignition circuit including a second capacitor connected in series rWith said ignitor electrode; ya discharge circuit including a saturable transformer coupling saidY charging circuit with said ignition circuit and periodical-lyoperatedswitching means to periodically transfer theienergy represented `bythe charge of said iirst capacitor onto said second capacitor, said saturable transformer being adapted to saturate a predetermined time interval 4after said second capacitor assumes its charge vthereby converting said ignition circuit into a low impedance discharge path permitting the flow of a pulse of ignition current through said ignitor electrode; and a bypass circuit branch connected across said second capacitor including a Vbypass rectifier poled to permit the flow of follow-up currents caused by the .residual inductance of said saturated transformer, whereby said pulse is characterized by a sharp rise of current followed by a relatively gradual decline of current which dissipates substantially yall of the energy stored in said ignition circuit.

8. In combination: an ignitron having an ignitor electrode in contact with a cathodic pool of mercury; means for impressing a source of alternating current potential across said ignitron; a charging circuit including a first capacitor connected to receive and store an electrical charge; a discharge circuit including a thyratron and a saturable transformer having a primary winding connected in series with said thyratron across said rst capacitor; means for periodically rendering said thyratron conductive thereby to discharge said rst capacitor through said discharge circuit; an ignition circuit including a second capacitor connected in series with said ignitor electrode across a secondary winding of said saturable transformer, whereby the periodic current discharge in said discharge circuit supports a periodic current flow in said ignition circuit to place a charge on said second capacitor, said saturable transformer being adapted to saturate a predetermined time interval after said second capacitor assumes its charge thereby converting said ignition circuit into a low impedance discharge path permitting the flow of an ignition current pulse through said ignitor electrode. Y

9. In combination: an ignitron having an ignitor electrode in contact with a cathodic pool of mercury; means for impressing 4a source of alternating current potenti-a1 across said gnitron; a charging circuit including a irst capacitor connected to receive and store an electrical char-ge; a discharge circuit including a thyratron and a saturable transformer having a primary winding connected in series with said thyratron across said first capacitor; means for periodically rendering said thyratron conductive thereby to discharge said trst capacitor through said discharge circuit; an ignition circuit including a second capacitor connected n series with said ignitor electrode across a secondary winding of s-aid saturable transformer, whereby the periodic current discharge in said discharge circuit supports a periodic current flow in said ignition circuit to place a charge on said second capacitor, said saturable transformer being adapted to saturate a predetermined time interval after said second capacitor assumes its charge thereby converting said ignition circuit into a low impedance discharge path permitting the low of an ignition current pulse through said ignitor electrode; and a bypass circuit branch connected across said second capacitor including a bypass rectier poled to permit the ow of follow-up currents caused by the residual inductance of said saturated transformer, whereby said pulse s characterized by a sharp rise of current followed by a relatively gradual decline of current which dissipates substantially all of the energy stored in said ignition circuit.

References Cited in the le of this patent UNITED STATES PATENTS 2,267,398 Edwards Dec. 23, 1941 2,320,790 Moyer et al June 1, 1943 2,517,129 Mulhern Aug. 1, 1950 2,624,040 Large Dec. 30, 1952 2,730,659 Hess Jan. 10, 1956 2,737,612 Sims Mar. 6, 1956 

